Medical practitioners, such as military medics, civilian emergency-medical personnel, nurses, and physicians, routinely perform vascular-access procedures (e.g., IV insertion, central venous-line placement, peripherally-inserted central catheter, etc). It is desirable for a practitioner to be proficient at performing these procedures since the proficient practitioner is far less likely to injure a patient and is almost certain to reduce the patient's level of discomfort.
Becoming proficient in vascular-access procedures requires practice. In fact, the certification and re-certification requirements of some states mandate a minimal number of needle sticks, etc., per year per provider. Historically, medical practitioners practiced needle-based procedures on live volunteers. More recently, simulation techniques and devices have been developed to provide training in vascular-access procedures without the use of live volunteers. U.S. Pat. No. 6,470,302 (“the '302 patent”) surveys the art of medical-simulation devices and also discloses a vascular-access simulation system.
The vascular-access simulation system that is disclosed in the '302 patent includes an “interface” device and a computer system. To practice a vascular-access procedure, a user manipulates an “instrument,” referred to in the patent as a “catheter unit assembly,” which extends from the device and serves as a catheter-needle. Potentiometers and encoders within the interface device track the motion and position of the instrument and relay this information to the computer system. The computer system performs a simulation of the surface and subsurface anatomy of human skin, and determines the effect of the instrument's motion on the skin's anatomy. Simulated results are displayed by the computer system. Using the motion information from the interface device, the computer system also generates a control signal that controls a force-feedback system that is coupled to the instrument. The force-feedback system generates various resistive or reactive forces that are intended to simulate the forces that are experienced by a medical practitioner during an actual vascular-access procedure. The user senses these forces during manipulation of the instrument.
The simulation system that is disclosed in the '302 patent has many shortcomings that substantially limit its utility as a training or accreditation tool. A few of these shortcomings are discussed below.
One shortcoming of that simulation system is that forces that are sensed by a user during manipulation of the catheter unit assembly are generally unrealistic. There are several reasons for this. One reason is that the linear axis along which the catheter unit assembly moves is offset from the rotational axes of a sensing/force-feedback assembly to which it's coupled. This results in an unrealistic torque sensation about the “insertion point” of the catheter unit assembly. A second reason for the unrealistic forces and force sensations that are experienced by a user is excessive friction. Specifically, the various tension members and bearings that couple the catheter unit assembly to the sensing/force-feedback assembly introduce a substantial amount of dynamic and static friction to the system. This is problematic because the interface device cannot present a force that is less than the friction that is inherent in the system. This excessive friction therefore limits the dynamic range of the system. Also, the presence of static friction (i.e., stiction) in the device hampers smooth motion of the catheter unit assembly. Stiction is not experienced during an actual vascular-access procedure.
A third reason for the unrealistic forces that are experienced during use of the device that is disclosed in the '302 patent is that the device has relatively high inertia. In particular, the large catheter unit assembly and the offset pulley used in the force-feedback mechanism introduce substantial mass into the system. This is undesirable because the catheter unit assembly will not feel as “light” as it should when little or no force feedback is being applied.
A second shortcoming of the '302 is that the end effector (i.e., the catheter unit assembly) is permanently coupled to the force-feedback system. Although not atypical for this type of system (i.e., haptics devices) due to the difficulty of de-coupling an end effector from its force-feedback system, this is very undesirable because to truly mimic most “actual” systems, de-coupling is necessary.
For example, in the case of an actual vascular-access procedure, a medical practitioner experiences “force-feedback” during insertion of a needle or catheter (i.e., an end effector) into a patient's arm. That is, the anatomy of the arm presents a resistance that is sensed (feedback) by the practitioner. In the actual procedure, the needle or catheter is not, of course, “coupled” to the arm until it is inserted by the practitioner. But in the system that is disclosed in the '302 patent, the catheter unit assembly is coupled to the force-feedback system and extends from interface device at all times. A user, therefore, does not actually insert the catheter unit assembly (i.e., the end effector); there is no coupling and de-coupling.
The inability of prior-art vascular-access simulation systems to realistically simulate a vascular-access procedure limits their usefulness as a training or accreditation tool.